Bone marrow transplantation (BMT) or hematopoietic stem cell transplantation (HSCT) is a medical procedure in the field of hematology and oncology that involves transplantation of hematopoietic stem cells (HSC). BMT and HSCT are most often employed in the treatment of patients suffering of diseases of the blood or bone marrow, or certain types of cancer. The objective of BMT or HSCT transplantation is to provide the patient with a healthy stem cell population that will differentiate into mature blood cells that replace deficient or pathologic cell lineages.
Hematopoietic stem cells are a rare population of cells within the bone marrow microenvironment. Hematopoietic stem cells actively maintain the continuous production of all mature blood cell lineages, which include major components of the immune system such as T and B Lymphocytes throughout life while maintaining a small pool of undifferentiated stem and progenitor cells (Mayani, 2003).
In the case of a bone marrow transplant (BMT), the HSC are removed from a large bone of the donor, typically the pelvis, through a large needle that reaches the center of the bone. The technique is referred as a bone marrow harvest and is performed with general anesthesia because literally hundreds of insertions of the needle are required to obtain sufficient material.
Peripheral blood stem cells (PBSC) are now the most common source of stem cells for HSCT. PBSC are collected from the blood through a process known as apheresis. The peripheral stem cell yield is boosted with daily subcutaneous injections of granulocyte colony-stimulating factor (G-CSF).
Another source of stem cells is umbilical cord blood. Cord blood has a higher concentration of HSC than is normally found in adult blood. However, the small quantity of blood that can be obtained from an umbilical cord (typically about 50 ml) makes this source less suitable for transplantation into adults. Newer techniques using ex-vivo expansion of cord blood units or the use of 2 cord blood units from different donors are being explored to facilitate cord blood transplants in adults.
During development, or in experimental and clinical transplantation, stem cells migrate through the blood circulation and home into the bone marrow (BM), repopulating it with immature and maturing myeloid and lymphoid blood cells, which in turn are released into the circulation. The process of hematopoietic stem cell homing and repopulation, which is crucial for stem cell function and development of the immune system, is not well understood.
In order to study the processes of hematopoietic stem cell homing and repopulation, several groups have established in vivo models including engraftment (incorporation of grafted tissue or cells into the body of the host) of human stem cells into immune deficient mice such as irradiated beige, nude, Xid (X-linked immune deficiency), SCID and non-obese diabetic SCID (NOD/SCID) mice, and in utero transplantation into sheep fetuses, which resulted in successful multilineage engraftment of both myeloid and lymphoid cells (McCune et al., 1988; Nolta et al., 1994; Lapidot et al., 1992; Larochelle et al., 1996; Civin et al., 1996).
The present inventors have developed a functional in vivo assay for primitive human SCID repopulating cell (SRCs) based on their ability to durably repopulate the bone marrow of intravenously transplanted SCID or NOD/SCID mice with high levels of both myeloid and lymphoid cells (Lapidot et al., 1992; Larochelle et al., 1996). Kinetic experiments demonstrated that only a small fraction of the transplanted cells engrafted and that these cells repopulated the murine bone marrow by extensive proliferation and differentiation. Furthermore, the primitive human cells also retained the capacity to engraft secondary murine recipients (Cashman et al., 1997). Transplantation of populations enriched for CD34 and CD38 cell surface antigen expression, revealed that the phenotype of SRC is CD34+CD38− (Larochelle et al., 1996). Other repopulating cells may exist since other studies suggest that immature human CD34− cells and more differentiated CD34+ CD38+ cells have some limited engraftment potential (Zanjani et al., 1998; Conneally et al., 1997).
Homing of human stem cells and their subsequent proliferation and differentiation in transplanted immune deficient mice was found to be dependent on interactions between chemokine stromal derived factor one (SDF-1), which is expressed by the host bone marrow, and its receptor CXCR4, which is expressed on the donor homing cells. Interfering with SDF-1/CXCR4 interactions by pretreatment of immature human CD34+ cells with neutralizing anti CXCR4 antibody blocked their in vivo homing and repopulation, while untreated cells could home within hours into the BM of recipient mice [Peled et al., 1999 (a)].
Increasing CXCR4 expression on the cell surface of stem cells, by cytokine stimulation, was found to enhance the response to SDF-1 of these cells manifested by improvement of homing and engraftment. Immature human CD34+ cells that do not express cell surface CXCR4 contain internal CXCR4, which can oscillate in vivo following transplantation. Prevention of this CXCR4 cell surface up regulation blocked the low levels of human CD34+ CXCR4− cell engraftment. Thus, the phenotype of repopulating human stem cells was defined as CD34+ CD38−/lowCXCR4+ cells (Kollet et al., 2002).
SDF-1 (also named CXCL12) is produced by many cell types including bone marrow stromal and endothelial cells, and as mentioned, is a powerful chemoattractant for immature and mature hematopoietic cells, and regulates leukocyte trafficking in steady state homeostasis. SDF-1 serves as a survival factor for stem and progenitor cells, and is involved in immature B cell and megakaryocyte development (McGrath et al., 1999; Nagasawa et al., 1996). SDF-1 is highly preserved throughout evolution. For example, human and mouse SDF-1 are cross-reactive and differ only in one amino acid.
Release and mobilization of stem cells from the bone marrow into the circulation are induced for clinical transplantation. Multiple stimulations with cytokines such as G-CSF are used to recruit human stem cells from the circulation. SDF-1/CXCR4 interaction within the BM following G-CSF administration was found to be involved in the mobilization process (Petit et al., 2002).
Proteolytic enzymes such as neutrophil elastase were found to degrade SDF-1 in the bone marrow during G-CSF administration. In parallel, the levels of CXCR4 expression on hematopoietic cells within the bone marrow were found to increase prior to their mobilization. Neutralizing antibody for CXCR4 or SDF-1 reduced human and mouse stem cell mobilization, demonstrating SDF-1/CXCR4 signaling in cell egress (Petit et al., 2002).
Thus, stem cell homing and release/mobilization utilize similar mechanisms, and in both processes SDF-1/CXCR4 interactions play a major role.
SDF-1 also plays an important role in the migration of leukemic cells. While normal and leukemic cells share similar mechanisms of migration, different homing patterns as well as SDF-1 signaling pathways were found when comparing malignant human Pre-B ALL cells (B-cell precursor acute lymphoblastic leukemia) to normal immature CD34+ cells (Spiegel et al., 2004). In acute myelogenous leukemia (AML), another malignant disease, high levels of intracellular CXCR4 and SDF-1 have been found in all leukemic cells, including cells that do not express surface CXCR4. CXCR4 is essential for the homing of these cells to the BM of immune deficient mice, demonstrating dynamic regulation of CXCR4 in these cells. (Tavor et al., 2005).
The expression of SDF-1 on the cell surface of endothelial cells within the blood vessels was found to be crucial for inducing cell arrest under shear flow, an essential step for a successful transendothelial migration from the circulation into the bone marrow. In addition, SDF-1 activated the major adhesion molecules such as CD44, LFA-1, VLA-4 and VLA-5 on migrating human stem and progenitor cells as part of the multistep process of homing and transendothelial migration (Peled et al., 2000). It has been suggested that SDF-1 mediates adhesion and anchorage of stem cells to the extracellular matrix of the BM niches by altering the cytoskeleton and relocating surface CD44 expression (Avigdor et al., 2004).
The mechanisms that induce cell motility and migration following SDF-1 stimulation and signal transduction pathways, which are triggered by binding of SDF-1 to CXCR4, are not known. Activation of PI3K, but not MAPK, has been found to be required for motility of enriched immature CD34+ cells. The atypical PKC zeta isoform was found to be essential for the process of migration. Moreover, activation of PKC zeta by SDF-1 was found to be PI3K dependent (Petit et al., 2005).
Beside its role in migration and adhesion, SDF-1 is also involved in proliferation and survival of various cells including normal human CD34+ cells and leukemic cells (Lee et al., 2002; Nishii et al. 1999, and Tavor et al., 2005).
Catecholamines are derived from the amino acid tyrosine. Catecholamines have a benzene ring with two hydroxyl groups, an intermediate ethyl chain and a terminal amine group (Lehninger, Principles of Biochemistry).
Catecholamines such as epinephrine (adrenaline), norepinephrine (noradrenaline) and dopamine may be regarded as derivatives of catechol or 1,2-dihydroxybenzene and they function as neurotransmitter substances (Lehninger, Principles of Biochemistry).
High epinephrine levels in blood are associated with stress, which can be induced from psychological reactions or environmental stressors. Epinephrine causes general physiological changes such as increases in heart rate, blood pressure, and blood glucose levels (Lehninger, Principles of Biochemistry).
Some drugs, like tolcapone (a central COMT-inhibitor), raise the levels of all the catecholamines (Wikipedia).
The adrenergic receptors (or adrenoceptors) are a class of G protein-coupled receptors that are targets of the catecholamines. Adrenergic receptors specifically bind their endogenous ligands, the catecholamines adrenaline and noradrenaline, and are activated by these (Wikipedia).
α-adrenergic receptors bind norepinephrine and epinephrine, though norepinephrine has higher affinity. Phenylephrine is a selective agonist of the a receptor (Wikipedia).
Dopamine is one of the three major catecholamine neurotransmitters in a variety of organs. Dopamine receptors have been widely established as key regulators of cardiovascular, renal, hormonal, central nervous system and ocular functions. In the brain, dysfunction of the dopaminergic system leads to Parkinson's disease and schizophrenia (Lim et al. 2005, Foley et al. 2004, and Goldman et al. 2004). Dopamine receptor subtypes belong to the family of G-protein-coupled receptors and share the characteristic structure of seven transmembrane domains. Five dopamine receptor subtypes can be classified into two families, referring to analogies in sequence and in signal transduction. The D1-like dopamine receptors include the dopamine D1 and the D5 receptors. They are characterized by activation of adenylyl cyclase mediated by a Gs protein, consequently effecting higher concentrations of the secondary messenger cyclic adenosine-3, 5-monophosphate (cAMP). The genes of the D1-like dopamine receptors lack introns. The D2-like receptor group consists of the dopamine D2, D3, and D4 receptors, which couple to Gi/0 proteins and can inhibit adenylyl cyclase. In the genes of dopamine D2-like receptors introns can be found (Sibley et al. 1992).
A role of different 7-transmembrane receptors, neurokinin-1 and 2, in hematopoiesis was recently reported. This study discloses that p53 partly regulates the anti proliferative effect of one of the ligands of NK2 receptor, the neurotransmitter neurokinin-A, on progenitor cells. This effect could be reversed by the cytokine GM-CSF (Vishalakumar et al. 2005).
A crosstalk between the immune system and the neuronal system was previously suggested, and neuro-immune interactions enable mutual regulation of the nervous and immune systems. Reports indicate that dopamine is one of the important mediators of neuro-immune interactions (Basu et al. 2000). Ilani et al (2004) suggested a pathway by which the brain affects and regulates immune activated T cells. Levite et al (2001) and Besser (2005) showed that dopamine could directly activate T cells via its specific receptors and suggest a possible role for dopamine in integrin-mediated trafficking and extravasation of T cells in the central nervous system and possibly in the periphery.